This invention relates to compositions of matter and methods of digesting wood chips used in paper pulping processes. The digestion is often achieved by chemical, mechanical or combined means. Chemical pulping is currently dominated in pulping industry, and among Kraft pulping is the most used pulping process. Chemical digestion is a process in which cellulosic raw materials such as wood chips are treated with chemicals including alkaline and sulfide for Kraft pulping or sulfites/bisulfites for sulfite pulping, usually at high pressure and temperature for the purpose of removing impurities and producing pulp suitable for papermaking. The mixture of chemicals is predominantly in a liquid form and is sometimes referred to as white liquor in Kraft pulping. Wood chips which consist primarily of cellulose, hemicellulose, lignin, and resins are broken down by digestion into a pulp of cellulose and hemicellulose fibers. The lignin and resins, which are undesirable in paper, are at least partially removed in the delignification stage of digestion.
The digestion process can be enhanced by the presence of one or more surfactants in the white liquor in Kraft pulping. The surfactants reduce the surface tension at the interface between the white liquor and the wood chips. This reduced surface tension allows the chemicals in the white liquor to penetrate more deeply into the wood chips and thereby better digest. Unfortunately the optimal composition of white liquor impairs the effectiveness of the surfactants. Because white liquor has a high pH, it causes most surfactants to salt out of solution especially in high temperatures and pressures. This reduces the amount of surfactant effective on the wood chips. Reducing the amount of surfactant causes wood chunks (known as rejects) to survive the digestion process which imposes additional costs and quality control issues in subsequent papermaking stages. Attempting to overcome this problem by supersaturating the white liquor with surfactant has been shown to offer little improvement and is undesirably expensive. Similarly, lowering the temperature, pressure, or pH of the white liquor, also results in more rejects surviving digestion.
Thus there is a clear need for, and utility in an improved method of digesting wood chips into paper pulp. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.